Commercial and industrial printing relies on a number of different printing methods for various purposes. Screen printing (or silkscreen printing) is a well-known method of printing wherein transfer inks or dyes are passed through a mesh stencil onto the substrate that is being printed on. The mesh stencil is formed to apply a single color over a single design area. After applying the stencil to the substrate, the colored ink is forced through the stencil, such as by a roller or squeegee passing over the stencil, onto the substrate. By applying different mesh stencils with different colors, a multicolored image can be printed onto the substrate. Screen printing provides a sharply defined edge for each color during the printing process.
Moreover, a wide variety of inks and dyes can be used in conjunction with screen printing. The most common inks today are plastisol inks, but specialty inks that provide different texture, vibrancy, or adherence to a substrate may be used for particular purposes. One such ink is metallic ink, which includes tiny metal particles suspended in the ink. The metallic particles provide a shimmery or effervescent appearance to the ink. Metallic inks are common in a variety of contexts and applications to provide such a metallic gleam. One such application is in the credit card industry, where metallic inks are printed on the plastic credit card substrate to provide a high-end look and feel that is valued by both banks and credit card users.
While screen printing provides high-quality images with a wide variety of ink colors and types, it also tends to be slower and more expensive than other common printing techniques. This is because screen printing requires a different mesh screen for each colored ink being applied to a printed substrate if the end product is to be multicolored. Moreover, the inks can only be applied one at a time. While it is well-known that the combination of only a few colors can result in a wide variety of colors, hues, and luminescence, such models require at least 4 colors to achieve any desired hue. (One commonly used color model of this type is cyan, magenta, yellow, and black (also known as “key”), and generally referred to as “CMYK.” This color model will be referred to in the description below, but the CMYK model can be converted into any number of other well-known color models, as known by a person of ordinary skill.) Therefore, in order to achieve truly multicolored images, a screen print must be repeated at least 4 times to achieve the desired colors. This results in a slower and more expensive process. For this reason, screen printing is typically only used for embellishments, such as metallic or pearl printing, while the basic 4-color models are applied using lithographic printing methods. However, because the wire mesh screens are static and must be changed out for any variation to be made when printing several cards, the screen printing process remains inefficient and time consuming. Additionally, for small-quantity runs, screen printing is very costly on a per-item basis. Accordingly, it is difficult to prepare custom images desired by consumers.
Other commercial and industrial printers address these perceived disadvantages with screen printing. Another type of commonly used printing is inkjet printing. In an inkjet printer, ink is forced through a microscopic nozzle in a “jet” or stream of ink. By forcing a microscopically thin jet of ink out the nozzle, the single stream of ink breaks into multiple small droplets of roughly the same size, the effect of a natural phenomenon known as Plateau-Rayleigh instability whereby fluids in streams break into smaller discrete units due to the surface tension of the fluid. Once the ink separates into microscopic droplets, the droplets may be directed onto the substrate to form an image.
While inkjet printers have a number of different methods of applying ink, one typical application process known as piezoelectric drop-on-demand printing is common in industrial-scale inkjet printing. In this method of printing, the inkjet nozzles release the individual colored inks only at specified locations as instructed by the computer printing software. (Hence, the colored ink is “dropped on demand,” rather than applied in a continuous stream.) The ink droplets are controlled by the flexing of a piezoelectric crystal in response to an electric field applied to the crystal. This allows very detailed and accurate control of the amount of ink dispensed in a particular droplet. Another form of drop on demand printing is thermal printing, whereby the nozzle is heated to expand to volume of the ink, thereby forcing ink out of the nozzle head.
When the ink contacts the substrate, it may be “pinned,” such that the ink solidifies, ceases flowing and is cured by UV rays. Additional drops of ink, which may either be the same or different colors, are added to the same spot on the substrate to create the desired color and hue. These are pinned and cured by UV rays in the same manner.
Because the inks can be applied simultaneously, inkjet printing can be significantly faster than screen printing techniques, and performed much more cheaply as well. However, inkjet printing also has particular disadvantages. One disadvantage is that not all inks can be applied, or applied as well, in inkjet printing. For example, metallic inks either clog the inkjet, or produce inferior improper images. In particular, the size of a metallic particle or flake that is necessary to produce the “sparkle” of the ink is too large to travel through an inkjet nozzle. Therefore, current metallic inks have much smaller flakes, and the resulting image takes on the appearance of a solid foil, rather than the desired sparkle. These images are also much slower to produce, and metallic inks for industrial inkjet printers are more expensive. Therefore, the use of metallic inks in inkjet printing presents too many obstacles to be a preferred and commonly used method in the mass production of credit cards.
If inkjet printing can achieve a metallic look and feel to an image without relying on metallic inks, inkjet printing could replace screen printing to achieve images of a similar look and quality, and do so more quickly and cheaply. What is needed, is a process for applying nonmetallic inkjet inks in a manner that mimics the look of a metallic ink applied in screen printing.